Novak Djokovic wins World Tour Finals after Roger Federer pulls out

(CNN) -- Roger Federer handed his fellow Swiss Stan Wawrinka a tough three-set loss in the semifinals of the World Tour Finals on Saturday. But it turns out the nearly three-hour epic also took something out of the 33-year-old.

Federer pulled out of the final against Novak Djokovic on Sunday, marking just the third time in his career he gave his opponent a walkover.

Federer went on court in a track suit to tell the crowd he wasn't match fit and later on Facebook revealed he hurt his back "late in the match yesterday."

He disclosed in a subsequent interview with the ATP it happened in the third-set tiebreaker of his late-night 4-6 7-5 7-6 (6) victory over Wawrinka.

"I felt all of a sudden the back was feeling funny," Federer told the ATP. "I tried to have treatment on it, medication on it, just tried to turn around as quick as possible really, but didn't really feel that much of an improvement overnight."

Federer was plagued by back issues in 2013, largely contributing to a subpar -- for him -- campaign. He had been in good health in 2014, though, one of the reasons the 17-time grand slam champion was able to climb the rankings and finish at No. 2.

"I tried all year to be ready for the ATP World Tour Finals, and I didn't want it to end this way," Federer stated to the crowd. "It would be too risky at my age to do this right now and I hope you understand.

"I wanted to come out personally and excuse myself. It's been a great week for me. I played some great tennis and I love coming to the O2 and to London, and there's been so many great memories for me here."

Djokovic thus becomes the first men's player since Ivan Lendl in 1987 to claim three consecutive year-end titles and his 31-match winning streak indoors remains intact.

He learned of Federer's withdrawal during the doubles final, which was scheduled 2 1/2 hours before the singles final.

"It's a very awkward situation to talk about," Djokovic told reporters. "You never like to win, especially these big matches against big rivals, with a (walkover)."

World No. 1 Djokovic stuck around to play an exhibition set against Andy Murray and that was followed by a doubles tussle featuring Murray and John McEnroe against Tim Henman and Pat Cash.

As big a blow as it was for Federer not to play, questions will now center around his fitness for the Davis Cup final against France in Lille that starts on Friday.

Indeed the Swiss team might be out of sorts come the final, since the Federer-Wawrinka encounter Saturday -- when Federer saved four match points -- became ill-tempered at times.

Wawrinka crunched a backhand at Federer when the latter was at the net and also, at one stage, Wawrinka jawed at the Federer camp, seemingly unhappy with something that was said to him.

The Davis Cup is one of the lone huge titles Federer has not won.

"Recovery obviously as quick as possible, and then traveling to France at some point and getting ready on the clay for the Davis Cup final," Federer said of his immediate plans.

Sunday's dramatic events brought an end to a tournament plagued by lopsided results in singles. Only four of the 14 singles matches extended to three sets.

"But this is something you cannot predict," said Djokovic, who handed out chocolates to journalists just as he did after winning last year's event.

The doubles competition featured plenty of excitement, including in the final when Americans Bob and Mike Bryan beat Croatia's Ivan Dodig and Brazil's Marcelo Melo 6-7 (5) 6-2 10-7.